Rotary joint for microwave energy



Nov. 28, 1961 v. J. ALBANESE ETAL 3,011,137

ROTARY JOINT FOR MICROWAVE ENERGY 3 Sheets-Sheet 1 Filed Sept. 19, 1960 n uu INVENTORS VICTOR .7. AL BANESE HER/WA N /(A GA N 4x1 M ATTORNEYS Nov. 28, 1961 v. J. ALBANESE ETAL 3,

ROTARY JOINT FOR MICROWAVE ENERGY Filed Sept. 19, 1960 3 Sheets-Sheet 2 H mm N I o INVENTORS Q 121% ATTORNEYS Nov. 28, 1961 V. J. ALBANESE ET AL ROTARY JOINT FOR MICROWAVE ENERGY Filed Sept. 19, 1960 3 Sheets-Sheet 3 INVENTORS.

V/CTDR J. ALBANESE HERMAN KAGA/V BY A TTORNEYS United States Patent,

3,011,137 ROTARY JOINT FOR MICROWAVE ENERGY Victor J. Albanese, Valley Stream, and Herman Kagan, Brooklyn, N.Y., assignorsto Bogart Manufacturing Corporation, Brooklyn, N.Y., a corporation of New York Filed Sept. 19, 1960, Ser. No. 56,958

15 Claims. (Cl. 333-98) .This invention relates to the transfer of microwave energy, and more particularly to a rotary joint for the same. P

In a typical instailation'a radar antenna rotates, and requires a rotary joint. In many cases two channels of different frequency may run through the same joint, it then being called a dual channel rotary joint.

The primary object of the present invention is to generally improve rotary joints for microwaves, and more particularly dual channel rotary joints. A more particular object is to provide a rotary joint capable of carrying large amounts of power, and more specifically, which avoids breakdown at the chokes associated with the rotary joint. Another object is to minimize VSWR (voltage standing wave' ratio) and to minimize wow.

Still another object is to devise means making it possible to use an oversize wave guide at the rotary joint, for example a B ban-d wave guide being used for X band energy with certain advantages, and without loss of efiiciency. v

Another object is to provide an improved dual-channel rotary joint'which is simpler in construction and cheaper to manufacture than heretofore, for the same performance requirements.

To accomplish the foregoing general, objects, and other more specific objects which will hereinafter appear, our invention resides in the rotary joint elements and their relation one to another, as are hereinafter more-particularly described in the fol-lowing specification. The specification is accompanied by drawings in which:

FIG. 1 is a longitudinal section through a dual channel rotary joint, embodying features of our invention;

FIG. 2 is an end view thereof looking toward the right end of FIG. 1;

FIG. 3 is a transverse section taken approximately in the plane of the line 33 of FIG. 1;

FIG. 4 is a fragmentary schematic view explanatory t of a modification;

.FIG. 5 shows a coaxial line portion to larger scale; 7

FIG. 6 is a section through a connector portion, drawn to large scale; and j FIG. 7 is a section through another connector portion, similarly drawn to large scale.

Referring to the drawing, and more particularly to FIG. 1', the rotary joint comprises a wave guide 12, and a coaxial line 14, 16 located on the axis of rotation of the joint, and transverse to the wave guide 12. A door knob transition 20 is disposed at the end of the coaxial line within the wave guide 12. This door knob is fixed to the inner conductor 16 of the coaxial 'line, and is rotatable relative to the wave guide. There is also a choke 22, 24 associated with and located outside of the door knob. Thus the door knob, instead of being stationary as heretofore, is rotatable of floating, and the choke, instead of being inside the door knob, is located wholly outside the door knob. The resulting large choke enables the rotary joint to handle greatly increased power, because the limiting factor usually is the breakdown voltage of the choke, and in the present case this is greatly increased by the large diameter of the choke.

. The part22 constitutes cylindricalclearance around the outer diameter of the door knob, and the part 24 is an annular choke chamber, which is disposed at the end,

knob, but it may be greater, and thisfact' is schematically illustrated in FIG. 4,131 which thedoor knob 26, as before, is secured to and'rotates with inner conductor 28. The cylindrical clearance 30 has adiarneter greater than that of the door knob, and the annular choke chamber 32 is located outside the clearance 30. The cylindrical portion 30 is about a quarter Wave long, in axial direction, as indicated, and the annular chamber 32 is dimensioned about one-quarter wave in radial dimension, again as indicated in the drawing.

In preferred form, wave guides are provided at both ends of the joint. Specifically there is not only a first wave guide 12 (FIG. 1), but also a second wave guide 34 which is rotatable relative to the first wave guide 12, and the coaxial line 14, 16 usually is perpendicular to the transition 36 within wave guide 34, but this door knob may be fixed to thewave guide 34 and to the coaxial line 14, 16.

Relative rotationtakes place at the first door knob 20, which therefore hasthe choke 22, 24 associated with it, while no comparable choke is needed at the door knob 36, which has no clearance or discontinuity, because there is no relative rotation.

Referring now to FIGS. 1 and 3 of the drawing, the wave guide 12 is terminated by a metal shorting plate 40 which is located remote from the axis of rotation, compared to usual practice. Specifically, it is located about one-half Wave length further from the axis than usual for such a shorting plate, which heretofore has been about one-quarter wave from the axis. Differently expressed, the shorting plate 40 is about three-quarter wave length from the axis of the joint, or it is about one-half wave length from the largest diameter part of the adjacent door knob 20.

Moreover, the wave guide 12 is oversized. In the particular case shown a B band wave guide having a width of 1.122 inches and a height of 0.497 inch is used for X band microwave, where the X band wave guide would be smaller, being about 0.9 inch wide and 0.4 inch high. The larger wave guide is capable of handling more power, and is operated in a region of its frequency range which is more remotely separated from the cut-01f frequency, in consequence of which the device is less frequency sensitive.

' These changes in dimension might be expected to cause some undesirable resonances. We have found that these may be overcome by stepping the wall surfaces of the wave guides inward, at the coaxial line, to a reduced dimension. -For structural convenience the stepped dimen- 'sion is obtained by usingrectangular metallic inserts, shown at 42 in FIG. 3,which are permanently secured in the side walls of the wave guide. They are located at points opposite the axis of the joint, to reduce or step the-dimension inward, as is clearly shown in FIG. 3. These inserts are used in wave guide 34 as well as in wave guide 12, there being four such inserts in all. In this. case they each step inward 0.060 inch, or 0.120 inch reduction in width for both sides.

As so far disclosed the rotary joint would handle only a single channel, but the design structure here employed has its greatest valuein a dual channel rotary joint, the otherchannel being carried by a coaxial line passing through :conductor 16.' More specifically, the conductor 16, which is the inner conductor of "coaxial line 14, 16,

Patented Nov. 28, 1961- ductor of another coaxial line. Referring to FIGS. 3 and 5, the inner conductor located on the axis of rotation is shown at 44, and appropriate insulation 46 fills the space between the conductors 44 and 16, which together constitute the inner coaxial line. The higher frequency channel is carried by the wave guides 12, 34 and the outer coaxial line 14, 16, while the lower frequency channel is carried by the inner coaxial line 16, 44.

Reverting to FIG. 1, there is a coaxial cable connector, generally designated 50, which is fixedly secured at one end of the tubular conductor 16. It is fixedly secured to the rotating structure as a whole, and therefore to the wave guide 34 and the door knob 36, as well as to the inner coaxial line 16, 44.

There is another coaxial cable connector generally designated 52, which is electrically connected to, but which is arranged to aiford relative rotation of the other end of the inner coaxial line.

The bearing load during rotation is taken mainly by a large ball bearing generally designated 54. The outer race of the ball bearing is received in a stationary bearing housing 56, and is locked in position in the housing by means of screws 58. The bearing housing 56 is mounted on a stationary structure receiving the same, as by means of a mounting flange 60 having holes 62 for mounting bolts. The mounting flange 60 here shown is eccentrical- 1y related to the axis of rotation, as will be clear from FIG. 2, but this was done to meet a particular mounting requirement, and has nothing to do with the invention. In most cases the mounting flange could be concentric with the axis of rotation.

The outer conductor of the coaxial line 14, 16 must separate or provide running clearance as between its rotating part 14 and its stationary part 56. This causes a discontinuity, and therefore requires a choke, and in the present case the choke is made up of an inner tubular portion 64 fittingwithin an outer tubular portion 66 sur rounded by an additional cylindrical chamber 68. These are preferably about one-quarter wave long, in accordance with known prior practise. The tubular part 64 is formed integrally with a base ring 70, and thetubular part 66 is formed integrally with a base ring 72, this being for convenience in structurally mounting the parts 64 and 66 in position in their respective stationary member 56 and rotating member 14.

There is also a running clearance at 74, and this is accordingly associated with a ring of lossy material 76-, which absorbs energy leaking through clearance 74, and thus minimizes reflection therefrom. In generally analogous fashion, the door knob 20 has an end clearance at 7 8 between it and the stationary member 80 which is fixedly mounted in main bearing housing 56. This clearance at 78 is associated with a ring or insert of lossy material 62. A running bearing may be provided at 84, this preferably being made of Fluorosint, which is sintered fiberglass and Teflon (polytetrafluoroethylene resin). The lossy material shown at 76 and 82, and at point 120 to be described later, may be a material sold commercially under the name Epocast, and is made of powdered carbonyl iron bound in an epoxy resin. Other bearing materials and lossy materials may be used.

The connector 50 may be conventional, and comprises a threaded part 86 which receives the inner threadof a mating connector (not shown) for connection to the outer conductor of a coaxial cable, perhaps a flexible cable. The part 86 of the connector is locked in member 92 by means of one or more set screws 94, and member 92 is fixedly secured to both the wave guide 34 and the rotatable bearing part 14. The inner conductor of the flexible coaxial cable, not'shown, terminates in a pin which is received in a longitudinally slit or resilient metal socket 88. This is spaced from and insulated from the outer conductor by a suitable insulation, in the case Teflon, shown at 90. The inner conductor and the insulation are tapered, as shown, to reduce them to the dimension used for the inner coaxial line 44, 16.

The door knob 36 may be made integral with the member 92, as shown, and these parts may be fixedly secured to tubular conductor 16 by means of a nut 96, the tubular part being reduced in diameter or stepped at 98 to fix the position of the door knob, and the endmost part being 99 receives the clamp nut. I

The center conductor 44 projects outward somewhat, as shown at 43. This is received in the longitudinally slit small end 87 (FIG. 6) of the conical metal piece 89. The insulation part 99 is locked in position by spinning a lip of metal inward, as shown at 91.

At its other end the coaxial line must be rotatable rela tive to the stationary connector 52 (FIG. 1). As here shown the connector is an elbow connector having an elbow shaped outer part 100, which is threaded at 102 to receive the inside thread of a mating female connector (not shown), thus providing electrical connection to the outer conductor of a coaxial cable, which may be a flexible cable. The inner conductor of the flexible cable terminates in a pin which is received in a longitudinally slit resilient sleeve 164 formed at the end of a bent inner conductor 106. The inner and. outer conductors are spaced by a suitable insulation 108, for example, Teflon. The body 160. is permanently secured, as by welding, to a plate 110, which in turn is removably secured to a stationary member 112 by means of mounting nuts 114, there being four as shown in FIG. 2.

The tubular conductor 16' (FIG. 1) is enlarged by means of a fitting 116. This is secured to conductor 16 by one or more set screws 118. The adjacent circular faces of the parts 112 and 116 provide a capacitive coupling for the outer conductor of the inner coaxial line. Because of the discontinuity provided by the necessary clearance between these relatively rotatable surfaces, we provide both a choke, and also lossy material. The ring or insert of lossy material is shown at. 121), and again may be Epocast or other lossy material. The choke is made up of clearance space at 122, and a choke chamber at 124-, these each being preferably about one-quarter wave long.

The inner conductor 44' of the inner coaxial line is expanded at 126 and is connected to the inner conductor 106 by means of a contact pin 128, which is spring pressed by means of a compression spring 130.

The construction is shown in greater, detail with reference to FIGS. 5 and 7, where the parts are shown drawn to larger scale. The metal outer conductor 16 (FIG. 5) is necked at 17. The center conductor projects outward somewhat, as shown at 45 (FIG. 5). Referring now to FIG. 7, the rotatable element 116 is counterbored at 117 to receive the necked portion 17 (FIG. 5). This in turn receives the set screw 118 (FIG. 7) previously mentioned. The projection 45 (FIG. 5)v of the small center conductor is received in the longitudinally slit small end 127 (FIG. 7) of the conical metal piece 126. This has an approximately' hemispherical seat 125, which in turn receives the spring pressed connector pin 128 (FIG. 1). The insulation part 129 (FIG. 7) is cemented to the parts 116 and 126.

In addition to the requirements already given, the rotary joint here shown is designed to be gas tight under pressure, and for this purpose it is sealed at a number of appropriate points by means of 0 rings. Such 0 rings are shown at 132, 134', 136, 138, 140, and 142 in FIG. 1.

The metal parts of the rotary joint may be made of a suitable aluminum alloy, when weight saving is a factor, or they may be made of ,brass when the extra weight is The part 97 receives the door knob, and the part unimportant. The spring pressed finger l2i8'is preferably made of stainless steel, while the parts 89,106, and 12;

may be made of beryllium copper.

In the particular case here shown the rotary joint is intended to handle a frequency range of 8.5 to 9.6 kmc. (in the X band) for the'hi'gher frequency, and a frequency range of fromlO to 1.2 kmc. (in the L band) for the lower frequency channel. nels is used both for transmission and reception in'radar work. The basic design may 'be usedfor other fre-' quencies by appropriate scaling of'dimensions.

The specifications or requirements met'by the present rotary joint are as follows: The VSWR is less than 1.15 ina frequency ra'ngefrom 855 to 9.6 km'c, The wow produces a VSWR variation of less than 1.03., T he'power capability in the X band channel is 300 watts'average Each of these chanradio frequencypower, and 300 kilowatts peak power at atmospheric pressure, andstillhigher when pressurized above one atmosphere. The insertion loss in the X band channel is less than 0.2 of a decibel. For the L- band channel the VSWR is less than 1.5 in a frequency range of from 1.0 to l.2 0-kmc., and with an insertion loss of less than, 0.2decibel. This is with HN coaxial connectors for terminating the L bandtransmission line, but for versatility these connectors mayfbe replaced with connectors of a different type, such as the N type connectors.

. One advantage of the present rotary joint is that there is greater tolerance in dimension, and in running fit. For example, if the door knobs vary slightly in dimension, or shift slightly in axial directi0n,'it does not affect the operation of the rotary joint nearly so much as has been the case in previous rotary joint designs.

Another feature concerns the' impedance match. The joint is. symmetrical, and because of its symmetry the following is possible. Each half is not impedance matched by itself. The halves are deliberately mismatched to give them an impedance characteristic'which' is conveniently matched-out by the other half, and this results in a broader band operation.

It isbelieved that the construction and operation of our improved rotary joint, as Well as the advantages thereof, will be apparent from the foregoing detailed description. Although the axis of rotation is shown horizontal in the drawing, it will be understood that the rotary joint may be disposed and used with its axis vertical or at'any desired angle. It will also be understood that while we have described one part of the joint as stationary and the other part as rotatable, the rotary joint is reversible, and either end could be stationary with the other rotatable. Similarly, =either endmay be used as aninput for microwave energy, and the other end as an output,

Although we show a rectangular wave guide/the term rectangular is not intended to exclude other wave guides such as a square wave guide. Although we show a door knob transition, the term is not'intended to exclude other high power transitions such as a conical transition or cylindrical transition. Here too the choke may be of larger diameter, as with the door knob.

It will beapparent that while we have shown and de- 'door knob transition being fixed to the inner conductor of the coaxial line but being rotatable relative to the wave guide, and a choke associated with the door knob, said y choke being outside of thedoor knob, the large diameter of the choke enabling the rotary'joint to handle a large amount of power without breakdown.

2. A rotary joint for microwave energy, said joint comprising a wave guide, a coaxial line disposed on the axis of rotation of the joint and transverse to the wave guide, fort-ransferring the microwave energy between the Wave guide and the coaxial line, a door knob transition at the end of the coaxial line within the wave guide, said door knob transition being fixed to the inner conductor of the coaxial line but being rotatable relative to the wave guide, there being a cylindrical clearance around the outer diameter of the door knob; an annular choke chamber having a diameter larger" than thedoor knob and disposed at the end of the cylindrical clearance around the door knob, said clearance andchamber-acting as a choke, the large diameter of said choke enabling the rotary joint to handle a large amount of power without breakdown. I

3. A rotary joint for microwave energy, said joint comprising a first wave guide, a second wave guide rotatable relative to the first wave guide on an axis transverse to the wave guides, a coaxial line disposed on the axis of relative rotation of the wave guides for transferring the microwave energy from one wave guide to the other, door knob transitions at the ends of the coaxial line within the Wave guides, one of said door knob transitions being fixed to its adjacent wave guide and to the inner conductor of the coaxial line, the other door knob transition being fixed to the said inner conductor-of the coaxial line but being rotatable relative to the adjacent wave guide,

and a choke associated with the door knob, said choke being outside of the door knob, the large diameter of said choke enablingthe rotary joint to handle .a large amount of power without breakdown. i

' 4. A rotary j'ointas defined in claim 3 in 'which the inner conductor of the aforesaid coaxial line which carries the door knobs-is tubular and acts as the outer conductor of another inner coaxial line the inner conductor of which extends along the axis of rotation, and in which thereis a coaxial cable connector fixedly secured at one end of said inner coaxial line for rotation therewith, and another coaxial cable connector electrically connected to but arranged to afford relative rotation ofthe other end of the aforesaid inner coaxial line,. whereb'y the rotary joint is adapted to handle dual channels with the higher frequency channel passing through the wave guides,an'd the lower frequency channel passing through'the coaxial cable connectors.

'5. A rotary joint for microwave energy, said joint comprising a first'wave guide,--a second-wave guide rotatable relativet'o the first Wavev guide on an axis transverse to the wave guides, a coaxial line disposed on the axis of relative rotation of the wave guides for transferring the microwave energy from one wave guide to the other, door knob transitions at the ends of the coaxial line within the wave guides, one-bf said door knob transitions being fixed to its adjacent waveguide and tothe inner conductor of the coaxial line, the other door-knob transition being fixed to the said inner conductor of the coaxial line but being rotatable relative to the adjacent wave guide,

there being a cylindrical clearance around the outer di-" ameter of the door knob,'- an annular choke chamber having a diameter larger than the door knob and disposed at the end of the cylindrical clearance around the door knob, said clearance and chamber acting as a choke, the large diameter of said choke enabling the rotary joint to handle a large'amount of power without breakdown.

6. A rotary joint for microwave energy, said joint comprising a'first waveguide, a 'second wave guide rotatable relative to the first wave guide on an'axis substantially perpendicular to the wave guides, acoaxial line disposed on the axis of relative rotation ofthe wave guides for trans-ferringthe microwave energy'from one wave guide to the other, door knob transitions at the ends of the coaxial line within the wave guides, one of said door knob transitions being fixed to its adjacent wave guide and to the inner conductor of the coaxial line, the other door knob transition being fixed to the said inner conductor of the coaxial line but being rotatable relative to the adjacent wave guide, there being a cylindrical clearance around the outer diameter of the door knob, said clearance being about a quarter wave long axially, an annular choke chamber having a diameter larger than the door knob and disposed at the end of the cylindrical clearance around the door knob, said chamher being about a quarter wave long in radial direction, said clearance and chamber acting as a choke, the large diameter of said choke enabling the rotary joint to handle a large amount of power without breakdown.

7. A rotary joint as defined in claim 6 in which the inner conductor of. the aforesaid coaxial line which carries the door knobs is tubular and acts as the outer conductor of another inner coaxial line the inner conductor of which extends along the axis of rotation, and in which there is a coaxial cable connector fixedly secured at one end of said inner coaxial line for rotation therewith, and another coaxial cable connector electrically connected to butarranged to aflord relative rotation of the other end of the aforesaid inner coaxial line, whereby the rotary joint is adapted to handle dual channels with the higher frequency channel passing through the wave guides, and the lower frequency channel passing through the coaxial cable connectors.

8. A rotary joint for microwave energy, said joint comprising a wave guide, a coaxial line disposed on the axis of rotation transverse to the wave guide for transferring the microwave energy between the wave guide and the coaxial line, a door knob transition at the end of the coaxial line within the wave guide, said door knob transition being fixed to the said inner conductor of the coaxial line but being rotatable relative to the adjacent wave guide, and a choke associated with and outside the door knob, the large diameter of said choke enabling the rotary joint to handle a large amount of power without breakdown, said wave guide terminating in a metal shorting plate, and the wall surfaces of the wave guide being stepped inward at the coaxial line to a reduced dimension.

9. A rotary joint for microwave energy, said joint comprising a wave guide, a coaxial line disposed on the axis of rotation transverse to the wave guide for transterring the microwave energy between the wave guide and the coaxial line, a door knob transition at the end of the coaxial line within the wave guide, said door knob transition being fixed to the said inner conductor of the coaxial line but being rotatable relative to the adjacent wave guide, there being a cylindrical clearance around the outer diameter of the door knob, an annular choke chamber having a diameter, larger than the door knob and disposed at. the end of the cylindrical clearance around the door knob, said clearance and chamber acting as a choke, the large diameter of said choke enabling the rotary joint to handle a large amount of power without breakdown, said wave guide terminating in a metal shorting plate disposed about three-quarter wave length from the axis of the joint, the. side walls of the wave guide having rectangular metallic inserts permanently secured therein at points opposite the axis of the joint to reduce or step inward the dimension of the wave guide at the rotary joint. I

10. A rotary joint for microwave energy, said joint comprising a. first wave guide, a second wave guide rotatable relative to the first wave guide on an axis transverse to the wave guides, a coaxial line disposed on the axis of relative rotation of the wave guides for transferring the microwave energy from one wave guide to the other, door knob transitions at the ends of the coaxial line within the wave guides, one of said door knob transitions being fixed to its adjacent wave guide and to the inner conductor of the coaxial line, the other door knob transition being fixed to the. said inner conductor of the coaxial line,

inner conductor of the aforesaid coaxial line which carries the door knobs is tubular and acts as the outer conductor of another inner coaxial line the inner conductor of which extends along the axis of rotation, and in which there is a coaxial cable connector fixedly secured at one end of said inner coaxial line for rotation therewith, and another coaxial cable connector electrically connectedto but arranged to afford relative rotation of the other end of the aforesaid inner coaxial line, whereby the rotary joint is adapted to handle dual channels with the higher frequency channel passing through the wave guides, and the lower frequency channel passing through the coaxial cab-1e connectors.

12. A rotary joint for microwave energy, said joint comprising a first wave guide, a second wave guide rotatable relative to the first wave guide on an axisttransverse to the wave guides, said wave guides being oversize relative to the frequency of the microwave energy being carried thereby, a coaxial line disposed on the axis of relativerotation of the wave guides for transferring the microwave energy from one wave guide to the other, door knob transitions at the ends of the coaxial line within the wave guides, one of said door knob transitions being fixed to its adjacent wave guide and to the inner conductor of the coaxial line, the other door knob transition being fixed to the said inner conductor of the coaxial line but being rotatable relative to the adjacent wave guide, there being a cylindrical clearance around the outer diameter of the door knob, an annular choke chamber having a diameter larger than the door knob and disposed at the end of the cylindrical clearance around the door knob, said clearance and chamber acting as a choke, the large diameter of. said choke enabling the rotary joint to handle a large amount of power without breakdown, said wave guides each terminating in a metal shorting plate disposed about three-quarter wave length from the axis of the joint, the wall surfaces of the wave guides being stepped inward at the coaxial line to a reduced dimension.

13. A rotary joint as defined in claim 12in which the inner conductor of the aforesaid coaxial line which carries the door knobs is tubular and acts as the outer conductor of another inner coaxial line the inner conductor of which extends along the axis of rotation, and in which there is a coaxial cable connector fixedly securedat one end of said inner coaxial line for rotation therewith, and another coaxial cable connector electrically connected to but arranged to afiord relative rotation of 'the other end of the aforesaid inner coaxial line, whereby the rotary joint is adapted to handle dual channels with the higher frequency channel passing through the wave guides, and the lower frequency channel passing through the coaxial cable connectors. 1

14. A rotary joint 'for microwave energy, said joint comprising a first wave guide, a second. wave guide rotatable relative to the first wave guide'on an axis substantially perpendicular to the wave guides, said wave guides being oversize relative to the frequency of the microwave energy being carried thereby, a" coaxial line disposed on the axis of relative rotation of the wave guides for transferring the microwave energy from one Wave guide to the other, door knob transitions at the ends of the coaxial line within the wave guides, one of said door knob transitions being fixed to its adjacent wave guide and to the inner conductor of the coaxial line, the other door knob transition being fixed to the said inner conductor of the coaxial line but being rotatable relative to the adjacent Wave-guide, there being a cylindrical clearance around the outer diameter of the door knob, said clearance being about a quarter wave long axially, an annular choke chamber having a diameter larger than the door knob and disposed at the end of the cylindrical clearance around the door knob, said chamber being about. a quarter wave long in radial direction, said clearance and chamber acting as a choke, the large diameter of said choke enabling the rotary joint to handle a large amount of power without breakdown, said wave guides each terminating in a metal shorting plate and the wall surfaces of the wave guides being stepped inward at the coaxial line to a reduced dimension which compensates for the oversize dimension of the wave guides.

15. A rotary joint as defined in claim 14 in which the inner conductor of the aforesaid coaxial line which carries the door knobs is tubular and acts as the outer conductor of another inner coaxial line the inner conductor 10 of which extends along the axis of rotation, and in which there is a coaxial cable connector fixedly secured at one end of said inner coaxial line for rotation therewith, and

another coaxial cable connector electrically connected to 10 cable connectors.

References Cited in the file of this patent UNITED STATES PATENTS Ragan June 14, '1949 2,476,732 Hollingsworth July 19, 1949 

